Discovery of the presence of LH/CG-R with germline and somatic activating mutations in patients with testicular tumor raised the question of its (mutated LH/CG-R) potential tumorigenic effect . Two patients with the activating mutation Asp578Gly mutation developed Leydig cell neoplasia, while a somatic Asp578His activating mutation was identified in a number of testicular tumor patients. Animal studies have so far failed to establish lines of male or female transgenic founder mice carrying LH/CG-R with the Asp578His mutation indicative of inherent differences between the germline and the somatic mutations. To explore the difference in the biological effects between the two mutations, a MA-10 Leydig cell model was established by stable transfection with the two mutant human receptors, LH/CG-R-Asp578Gly and LH/CG-R-Asp578His. Examination of gene expression profiles using cDNA microarray and a systems biological approach, hierarchical clustering and multi-dimensional scaling analysis , the wild type and the two mutants could be distinguished . The expression of 54 genes was altered by the presence of LH/CG-R Asp578Gly mutation, while 49 genes were changed in the presence of the LH/CG-R Asp578His mutation. By comparing both mutants to the wild type, 132 genes were found to be differentially expressed. Novel regulatory pathways unique to each mutation were identified; 9 networks in LH/CG-R-Asp578Gly expressing cells, and 12 in LH/CG-R-Asp578His expressing cells. Further analyses showed c-Myc and c-Src to be the key regulators associated with Asp578Gly and Asp578His mutants, respectively. The involvement of these two factors was confirmed by molecular and functional assays. The results open a new dimension and provide a novel explanation for the role of LH/CG-R mutation in testicular tumorigenesis.